Two-dimensional drawing devices which provide a planar window upon which sketches may be formed by the manipulation of controls which position a scribe to remove a powder material from the lower or inner surface of the planar window are well known. One such device is the popular ETCH-A-SKETCH (a registered trademark of the Ohio Art Company).
A two-dimensional drawing device of the ETCH-A-SKETCH type is disclosed in U.S. Pat. No. 3,055,113 issued to Grandjean. This device uses a system of pulleys and cables to position a scribe along the lower surface of a transparent planar window. Two knobs control the cables and pulleys. Rotating one knob causes the scribe to move horizontally while rotating the other knob causes the scribe to move vertically. Therefore, various patterns and figures can be formed upon the planar window by manipulating the two knobs to move the scribe across the lower surface of the planar window. As the scribe moves across the lower surface of the planar window it scrapes the powder material away, thereby forming an image.
By inverting the two-dimensional drawing device and shaking it moderately, the sketch so formed can be erased. A quantity of powder material is disposed within the housing which acts as a reservoir for the powder material. The housing also provides structural support for the pulleys, cables, control knobs, and transparent planar window. Inverting the device causes the powder material, which normally rests upon the floor of the housing, to come into contact with the inner surface of the transparent planar window. Shaking the device moderately assures that the powder material will adhere to all areas of the planar window, particularly where the powder material has been removed by the scribe.
An obvious limitation of the two-dimensional drawing device is that it can only be used to form sketches upon a planar surface. This restricts the use of the two-dimensional drawing device to images that are well represented in two dimensions. As is well known, not all images can be suitably represented in two dimensions.
The surface of a globe, such as the earth's surface, cannot be represented adequately in two dimensions. Attempts to do so have resulted in maps such as the Mercator Projection in which the meridians or lines of longitude are drawn parallel to each other and the parallels or lines of latitude are drawn as straight lines whose distance from each other increases with their distance from the Equator. This, of course, results in a gross distortion of the true nature of the earth's surface, which can only be properly represented upon the surface of a sphere.
U.S. Pat. No. 4,364,176 issued to Cassagnes et al. discloses a drawing device which has a curved drawing surface. The drawing surface of the Cassagnes device is convex toward the user, providing an outward bulge much like the screen of a television set. While the screen of the Cassagnes device is not technically two dimensional, since it is not planar, it is limited to depicting much the same type of images. The screen of the Cassagnes device is essentially nothing more than a planar screen with a slight bulge in the center.
As such, although the prior art has recognized to a limited extent the problems of properly representing three-dimensional images, the proposed solutions have to date been ineffective in providing a satisfactory remedy.